The invention is directed to a method and to an arrangement for transmitting information in a digital radio system. The invention is also directed to a central station and to a subscriber station of a digital radio network.
The digital, cellular pan-European mobile radio system GSM (Global System for Mobile Communication) is an example of a digital radio system. In this system, the multiple access method TDMA (Time Division Multiple Access) is used, whereby the mobile radio telephone subscribers are differentiated by different time slots in a time-division multiplex system. The GSM is standardized for the 900 MHz range (GSM 900) and for the 180 MHz range (DCS 1800). In the GSM 900, the base stations receive and the mobile stations (MS) transmit in the frequency band from 890-915 MHz (referred to as bottom band), and the base stations transmit and the mobile stations receive in the frequency band from 935-960 MHz (referred to as top band). Further, one can also operate in an additionally specified frequency band G1 in the GSM 900, whereby the mobile stations transmit and the base stations receive in the top band from 880-890 MHz and the base stations transmit and the mobile stations receive in the bottom band from 925-935 MHz.
A top band from 1710-1785 MHz is provided for the DCS 1800 system wherein the mobile stations transmit and the base stations receive, and a bottom band from 1805-1880 MHz is provided wherein the base stations transmit and the mobile stations receive.
The frequency band width of a carrier (carrier frequency) in the GSM amounts to 200 kHz and each carrier having the band width of 200 kHz is divided into eight time slots. The GSM is thus a combination of the frequency-division multiple access method (FDMA) and the time-division multiple access method (TDMA). In this case, one speaks of an FDMA/TDMA system.
One data channel or one voice data channel is allocated to every mobile radio telephone subscriber in the GSM for the transmission of data or of digitized speech, whereby one data channel or one voice data channel is allocated to a time slot of a carrier. In order to protect the data stream against disturbances on the radio channel, a forward error correction FEC is implemented. Redundancy is thereby attached to the information (data or digitized speech) to be transmitted with the assistance of a channel coding. The attached redundancy serves the purpose of recognizing and correcting errors. The maximum gross transmission rate of a time slot that is composed of the information to be transmitted and of the attached redundancy amounts to a maximum of 22.8 kbit/s.
In the GSM, the digitized speech is transmitted with a net transmission rate of 13 kbit/s. 9.8 kbit/s redundancy is attached with the assistance of the channel coding for the error-protection of the digitized speech. In addition to the voice channels, there are channels in the GSM with different useful data rates, whereby, however, the maximum gross transmission rate, as already mentioned, amounts to 22.8 kbit/s. The channels existing in the GSM with the useful data rates of 12.0 kbit/s, 6.0 kbit/s or, respectively, 3.6 kbit/s have a redundancy of 10.8 kbit/s, 16.8 kbit/s or 19.2 kbit/s attached to them by the channel coding, so that a gross transmission rate of 22.8 kbit/s always results.
One disadvantage of this system is that a maximum error-recognizing and error-correcting transmission rate of only 12.0 kbit/s can be assigned to a radio telephone subscriber in the GSM in view of the transmission. If the redundancy were omitted, then a maximum, redundancy-free transmission rate of 22.8 kbit/s could be achieved in the GSM. The disadvantage given the omission of the redundancy, however, is that disturbances occurring during transmission, which result in bit errors, cannot be recognized and/or corrected.